The research of soft matter properties by light scattering material adding drop additive

2.1 Agents

MMA, St and BPO (initiator, benzoyl peroxide) are chemical pure agents used in experiments. MMA (Institute of Chemical Reagent, Tianjin, China) and St (North Tianyi Chemical Reagent Factory, Tianjin, China) were distilled at reduced pressure and purified before use. BPO (Institute of Chemical Reagent, Tianjin, China) was purified by being dissolved in chloroform, precipitated in methanol and dried in vacuum.

2.2 Sample preparation

To begin with, styrene (St) pre-polymer was prepared by thermal polymerization method. Two percent of the initiator, BPO and 0.4%–1% of St were added to a dry flask with stirring. The pre-polymerization was carried out in a water bath at the constant temperature of 80°C for 10 min. After that, the pre-polymer was mixed with MMA monomer (with 2% BPO and 0.2% α-hydroxylethyl ferrocene) (Weida Chemical Co., Ltd., Beijing, China) and stirred for another 30 min before it was cooled down to room temperature (about 30°C) and reacted until solidification.

2.3 Characterization

The optical properties of the samples – transmittance and haze – were measured by using a WGT-S transmittance/haze instrument. The light scattering property of the material was characterized by the haze of the samples. Microstructure of samples was examined by XL30 S-FEG field emission scan electronic microscope. IR spectra were measured by NICOLET-210 FT-IR instrument.

3.1 Effect of additive substance on polymeric reaction

The mixture of ferrocene and BPO can produce the charge transfer complex (CTC), which could decompose rapidly and initiate polymerization of PMMA more efficiently by producing free radical under room temperature [13]. Therefore, we synthesized α-hydroxylethyl ferrocene, which is the derivative of ferrocene. Then we added it to the MMA/BPO system. The temperature of in situ polymerization of PMMA can decrease from 85°C to room temperature (12–35°C). Therefore, the addition of α-hydroxylethyl ferrocene provides a potential method to simplify the preparing process of polymeric light scattering material. Additionally, it favors seeing about characteristic of PMMA as soft matter [14–18].

3.2 Effect of the initiator

The scattering PS is distributed in PMMA to introduce light scattering property. Different molecular weights can lead to different light scatter behaviors. By controlling the concentration of BPO, the molecular weight of PS can be controlled [19] and, therefore, the transmittance [20] and haze [21] of the sample can be modified. The relationship of the transmittance and the haze of the materials at different concentrations of BPO but same pre-polymer (St) concentration is presented in Figure 1. It can be found that both transmittance and haze have a peak value with respect to the increasing concentration of BPO. The maximum value of haze is 30% more than the minimum, but the change of transmittance is only within 1%.

Figure 1:

The relation between the concentration of BPO and the optical performance of the sample (The concentration the pre-polymer is 0.2%).

3.3 Effect of the pre-polymerization time

Another aspect to control the PS conversion rate is by controlling the pre-polymerization time. Different pre-polymerizing times can bring in different molecular weights and different scattering intensities of PS. From Figure 2, it can be found that with the increasing pre-polymerizing time, transmittance and haze of the sample decreased in oscillation.

Figure 2:

The relation between the pre-polymerization time and the optical performance of the sample (the concentration of prepolymer is 0.2%, the pre-polymerization temperature is 80°C, the concentration of BPO is 0.5%, the sample is measured after 47 days in room temperature).

3.4 The effect of the pre-polymer(St) concentration on the rate of polymerization

The pre-polymer (St) and MMA start to polymerize when heated to 80°C. The time when the polymerization accelerates is named as T. It can be seen from Figure 3 that when the concentration of pre-polymer (St) increases, T is longer and the rate of polymerization decreased. The reason is that with the increase of the pre-polymer concentration, the remaining St monomer contained in the MMA increased. The reaction activation energy of St is larger than that of MMA. So the rate of polymerization decreases. On the other hand, the concentration of PS is small, so the gel effect is not obvious and it has little effect on the polymerization. In summary, the higher the concentration of pre-polymer, the slower the polymerization rate. From our experiment, it can be found that high concentration of pre-polymer leads to significant decrease of the transmittance of the sample, which cannot be used as light scattering material. When the concentration of the pre-polymer is 10%, the transmittance decreases to 40%. We also find that when the concentration of the pre-polymer is 20%, the polymerization solidification phenomenon cannot be observed in 180 days, which suggests that polymerization cannot occur in this situation.

Figure 3:

The relation between the pre-polymer concentration and the rate of polymerization (the pre-polymer time is 20 min; the pre-polymer temperature is 80°C; T is the starting time of pre-polymerization acceleration).

As shown in Figure 4, with the increase of the pre-polymer (St) amount, the transmittance of the sample decreases and the haze of the sample increases. The monomer St included in the pre-polymer could copolymerize with MMA to form P(MMA-St) because of the reactivity ratio. Therefore, there are PMMA, P(MMA-St) and PS in the sample eventually. As P(MMA-St) has the same characteristic with hydrophilic oil wet of surfactant, it has good compatibility with both PMMA and PS, which can solve the problem of reduction of brittleness and impact resistance as well as improve the transmittance and haze of the sample.

Figure 4:

The effect of the prepolymer (St) concentration on the optical performance of the sample.

3.5 The effect of the prepolymer (St) concentration on the light scattering coefficient

The ability of light scattering is characterized by the light scattering coefficient, the product of transmittance and haze. It shows the capability of the material to scatter light without light loss [22]. The light scattering coefficient of the sample with the additive of BPO (0.5%) and prepolymer (various from 0 to 10%) is shown in curve B of Figure 5. It can be seen that the optimum concentration of prepolymer (St) was 0.5%–1%. The proper process condition is prepolymerization at 80°C with concentration of BPO at 0.5% for 20 min and then mixing 0.5% prepolymer with 99% MMA.

Figure 5:

The effect of α-hydroxylethyl ferrocene concentration on the light scattering coefficient. Curve B – sample without adding α-hydroxylethyl ferrocene, curve C – sample adding α-hydroxylethyl ferrocene.

If the polymerization temperature can be reduced from 80°C to room temperature, it would bring in great impact on the energy consumption during the synthesis. With the addition of just 0.2% α-hydroxylethyl ferrocene, this goal is successfully achieved. The light scattering coefficient of the sample after adding α-hydroxylethyl ferrocene is shown in curve C of Figure 5. It could be seen that the light scattering coefficient decreases after adding α-hydroxylethyl ferrocene. It is because the composite material turns into orange, which would absorb the light.

As shown in the experiment, the addition of α-hydroxylethyl ferrocene would prevent polymerization when polymerizing St alone. So the additive amount of α-hydroxylethyl ferrocene and pre-polymer (St) should be kept at low concentration in order to reduce the polymerization temperature of MMA. As shown in curve C of Figure 5, there is limited effect on the light scattering coefficient when the additive of pre-polymer (St) is <0.05%. But when the additive of pre-polymer (St) is 10%, the light scattering coefficient decreases to 40%.

3.6 Molecular weight measurement

The molecular weight distribution curve of MMA with prepolymer (St) is shown in curve A of Figure 6. It can be seen that in the distribution of molecular weight, the molecular weight polydispersity is wider compared with PMMA synthesized by using conventional ways. This wider molecular weight distribution, caused by the copolymerization of MMA/St, leads to better light scattering property. The molecular weight polydispersity can be seen in curve B of Figure 7.

Figure 6:

The test pattern of the molecular weight. Curve A – sample without adding α-hydroxylethyl ferrocene, curve B – sample adding α-hydroxylethyl ferrocene.

Figure 7:

The relationship between molecular weight distribution and the concentration of prepolymer (St). Curve B – sample without adding α-hydroxylethyl ferrocene, curve C – sample adding a-hydroxylethyl ferrocene.

The molecular weight distribution curves of MMA with prepolymer (St) and α-hydroxylethyl ferrocene are shown in curve B of Figure 6. It can be seen that the distribution of molecular weight is wider and the polydispersity is bigger than the one without α-hydroxylethyl ferrocene, as shown in curve C of Figure 7.

The molecular weight of the sample as a function of changing prepolymer concentration is shown in Figure 8, while the molecular weight distribution of the sample is shown in Figure 7. In Figures 7 and 8, curve B represents the additive of prepolymer (St) and curve C showed the additives of prepolymer (St) and α-hydroxylethyl ferrocene. It can be seen from curve C that the molecular weight and the molecular weight distribution have a maximum. The reason for the occurrence of the maximum is more production of copolymer (St-MMA) with the increasing amount of pre-polymer (St). But PS in the pre-polymer would attract the copolymer and the steric hindrance would prevent the progress of copolymerization if the pre-polymer (St) continued to increase. The molecular weight distribution value of the sample with pre-polymer is from 3.0 to 3.3, and this value increases to 4.5–5.5 after adding α-hydroxylethyl ferrocene.

Figure 8:

The relationship between molecular weight and the concentration of prepolymer (St). Curve B – sample without adding a-hydroxylethyl ferrocene, curve C – sample adding a-hydroxylethyl ferrocene.

Figures 6–8 show the molecular distribution in a wide range, which could scatter the light of different wavelength. The characteristic of soft matter (PMMA) can be obtained through the addition of a little amount of prepolymer (St) and α-hydroxylethyl ferrocene, leading to a large change in light scattering and molecular weight.

3.7 FT-IR spectra analysis

Figures 9 and 10 show the FT-IR spectra of the sample without and with adding α-hydroxylethyl ferrocene, respectively. As shown in Figure 11, the strongest absorption peak with a wave number of 1730.84 cm^-1 shows C=O band flexural vibrations; the absorption peaks with wave numbers of 1271.43 cm^-1, 1240.96 cm^-1, 1192.30 cm^-1, and 1148.82 cm^-1 show C-C-O-C band flexural vibrations; and the absorption peak at 2996.39 cm^-1 and 2950.72 cm^-1 indicates C-H band flexural vibrations. All of the abovementioned peaks are the typical absorption peaks of PMMA [23]. The typical absorption peaks in Figure 9 are consistent with the one in Figure 10. While in contrast with Figure 9, absorption intensity of typical peaks in Figure 10 decreases in various degrees. The peak of wave number of 909.09 cm^-1 in Figure 9 moves to 912.16 cm^-1 in Figure 10. The reason for the appearance of this peak is either the charge transfer leading to the vibration of polymer side chain or the formation of new group function.

Figure 9:

IR spectra of the sample without additive substance.

Figure 10:

IR spectra of the sample containing additive substance.

Figure 11:

SEM microstructure of the sample without additive substance.

Generally, when IR spectra instrument is used in measurement and analysis, the utmost limit of the detection of one substance from the other is about 5% [24]. As the concentration of pre-polymer (St) is very low, it is hard to get valuable information through infrared spectrum for sample structure testing. So it is reasonable that we are not able to observe the typical peaks of PS and α-hydroxylethyl ferrocene. However, surprisingly, we can find a small typical peak of hydroxyl at the wave number of 3434.55 cm^-1. We believe that α-hydroxylethyl ferrocene carried out oxidation and deoxidization reaction with BPO, existing in matrix in the form of ion and reinforced the eigen vibration of hydroxyl.

3.8 SEM analysis

As shown in Figure 11, in a compound material without adding α-hydroxylethyl ferrocene, many particles are dispersed in consecutive phases. The size range of particles is wide, from dozens of nanometers to several hundred nanometers. Undoubtedly, these particles are PS uniformly dispersed in PMMA matrix as scattering substance. The size of scattering particles is in accordance with that of the visible light wavelength or less, so the material shows strong light scattering. On the other hand, the even distribution of scattering particles and their various sizes make the material scattering light uniformly. These properties are well presented in the optical measurement.

By comparing Figures 11 and 12, it can be seen that distribution of dispersed phase in matrix changes after adding α-hydroxylethyl ferrocene. Dispersed phase does not exist in the form of particle distributed in consecutive phase anymore. It forms consecutive single chips with flower shape dispersed in consecutive phase, as shown in Figure 13A. Some of these chips have central particles, while others have none. They are arrayed in a cascade-like shape. There are many smaller particles distributed in the face of a chip (Figure 13D). Because the length of the boundary (Figure 13B) and the size of central particle (Figure 13C) are larger than the visible light wavelength, transmittance of the material reduces. Furthermore, as the size of particle widely dispersed in the face of chips is obviously smaller than the scale of the visible light wavelength, strong light scattering cannot occur [25, 26].

Figure 12:

SEM microstructure of the sample containing additive substance.

Figure 13:

SEM microstructure of the single chip. (A) Whole Single chip, (B) Boundary, (C) Central particle (D) Surface of the single chip.

3.9 Model to calculate the effect of the addition of pre-polymer (St) on the transmittance and haze

After measuring the particle quantity and the particle dimension, we can get the haze and transmittance of PMMA/PS light scattering material.

The particle quantity and the particle dimension can be obtained from Figure 11. The average dimension of the particle d̅ can be calculated by the following equation:

The haze of the light scattering material H can be determined by

And the transmittance of the material T is equal to

where j is the shape coefficient, p stands for the weight of skew rays (deviation angle is larger than 2.5°) in all rays through the PS particle, and k stands for the weight of reflected rays in all rays through the PS particle. The value of p and k can be obtained from the ray tracing model established by Matlab software, as shown in Figure 14, which is based on Snell’s law in vector form [27].

Figure 14:

The ray tracing model of PS particle.

where n is the refractive index of the first media, n′ is the refractive index of the second media, A is the unit vector of the incident ray, A′ is the unit vector of the refracted ray, N⁰ is the unit vector of the normal of the incident point, and Γ is the bias constant.

Then we can calculate that the haze of the sample without additive substance is 46.3% with an error of 7.4% and the transmittance is 93.0% with an error of 2.9%.

The effect of the content of prepolymer (St) on transmittance of the sample can also be simulated by the model. The simulated and the experiment values are shown in Figure 15. The mode successfully predicts the experiment result by a 4.5% difference.

Figure 15:

The effect of the content of prepolymer (St) on transmittance of the sample.